In the power generation industry, steam turbines are critical for converting thermal energy into mechanical work. Among their key components, the Intermediate Pressure (IP) outer cylinder is paramount. Due to its massive size and complex geometry, it is typically manufactured via casting using alloy steels such as ZG15Cr1Mo1 or ZG15Cr2Mo1. However, the intricate casting process for these large components invariably introduces internal discontinuities. The presence of casting defect like micro-cracks, gas porosity, shrinkage cavities, and slag inclusions is a common and significant challenge. These casting defect are often discovered during rough or finish machining stages on surfaces such as end faces and outer walls. Left unrepaired, these flaws compromise the structural integrity, operational safety, and service life of the cylinder, while also causing substantial delays in production schedules. Therefore, developing a reliable, qualified welding repair procedure is not merely a corrective action but a fundamental necessity for ensuring product quality and economic viability. This article details a first-person perspective on the systematic approach—from material analysis and procedure qualification to final application—for successfully addressing these casting defect.
The selection of ZG15Cr1Mo1, a 1.25%Cr-1%Mo type heat-resistant cast steel, is driven by its requirements for high-temperature strength and pressure containment. However, its alloying elements also dictate specific welding characteristics that must be meticulously managed. A primary tool for assessing weldability is the carbon equivalent (CE) formula, which helps predict the hardness and susceptibility to cold cracking. Using a standard formula for heat-resistant steels, the carbon equivalent for ZG15Cr1Mo1 is calculated as follows:
$$ CEV = C + \frac{Mn}{6} + \frac{(Cr+Mo+V)}{5} + \frac{(Ni+Cu)}{15} $$
Substituting typical values for ZG15Cr1Mo1 (C~0.15%, Mn~0.45%, Cr~1.25%, Mo~1.0%, with V, Ni, Cu being low), the calculation yields:
$$ CEV \approx 0.15 + \frac{0.45}{6} + \frac{(1.25+1.0)}{5} \approx 0.15 + 0.075 + 0.45 = 0.675 $$
It is widely recognized in welding metallurgy that a CEV value exceeding 0.45 indicates a significant tendency for hardening during welding, especially under conditions of high restraint and rapid cooling. This hardening promotes the formation of hydrogen-induced cold cracks. Consequently, any welding repair protocol for this casting defect must incorporate stringent measures to mitigate this risk. These measures include mandatory preheating to slow the cooling rate, strict control of interpass temperature, and the application of post-weld heat treatment (PWHT) for hydrogen diffusion and stress relief. Furthermore, the heat input during welding must be carefully controlled. Excessive heat input can lead to grain coarsening in the heat-affected zone (HAZ), reducing toughness and potentially increasing susceptibility to reheat cracking—another concern for Cr-Mo steels. Thus, the welding philosophy must balance the need to prevent cold cracks (via sufficient heat) with the need to preserve microstructure (via limited heat).
Before any repair is attempted, a thorough understanding of the base metal’s guaranteed properties is essential. The cylinder material, conforming to an internal specification, was subjected to chemical analysis and mechanical testing. The results, compared against the standard requirements, are summarized below. The tight control of carbon, sulfur, and phosphorus is notable, as these elements directly impact weldability and fracture toughness. The deliberate control of chromium and molybdenum near the upper specification limit ensures the desired elevated temperature strength.
| Material / Condition | C | Mn | S | P | Si | Cr | Mo | Rm (MPa) | ReL (MPa) | A (%) | HBW |
|---|---|---|---|---|---|---|---|---|---|---|---|
| ZG15Cr1Mo1 Spec. | 0.10-0.18 | 0.20-0.50 | ≤0.012 | ≤0.020 | 0.20-0.60 | 1.00-1.50 | 0.90-1.20 | ≥550 | ≥345 | ≥18 | 170-220 |
| Test Sample (38mm) | 0.13 | 0.44 | 0.001 | 0.011 | 0.35 | 1.40 | 1.13 | 618 | 459 | 27 | 193 |
The choice of welding process is critical for the efficiency and quality of repairing extensive casting defect. While Shielded Metal Arc Welding (SMAW) is traditionally used for such repairs, Gas Metal Arc Welding (GMAW) with a flux-cored electrode offers distinct advantages for this application. The Flux-Cored Arc Welding (FCAW) process, utilizing an 80%Ar-20%CO₂ shielding gas mixture, provides higher deposition rates, improved operator comfort due to less physical effort, and potentially better weld metal quality with lower hydrogen potential compared to some manual electrodes. This leads to increased productivity—a vital factor when repairing large-volume casting defect—while maintaining the required metallurgical integrity.
Selecting the appropriate filler metal is a cornerstone of the repair strategy. The principle is to match or slightly over-match the base metal’s composition and high-temperature performance. For the 1.25Cr-1Mo base metal, a 2.25Cr-1Mo type filler metal is often chosen. This slight overalloying in chromium content helps compensate for any potential dilution and ensures the weld metal’s creep strength is adequate. The selected filler was TWE-911B3M (E621T1-B3M classification), a 1.2mm diameter flux-cored wire. Its chemical composition and mechanical properties, as verified by testing, are presented below. The results confirm that the weld metal meets the required specification, with excellent strength and good ductility.
| Wire / Classification | C | Si | Mn | P | S | Cr | Mo | Rm (MPa) | ReL (MPa) | A (%) | Impact Energy (J @ 10°C) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| E621T1-B3M Spec. | 0.05-0.12 | ≤0.80 | ≤1.25 | ≤0.03 | ≤0.03 | 2.00-2.50 | 0.90-1.20 | 620-760 | ≥540 | ≥17 | – |
| TWE-911B3M Test | 0.058 | 0.207 | 0.866 | 0.016 | 0.012 | 2.09 | 1.02 | 705 | 640 | 18.5 | 73 (avg) |
Prior to executing repairs on the actual component, a formal Welding Procedure Qualification Record (WPQR) was developed in accordance with the ASME Section IX standard. This is a non-negotiable step to scientifically validate that the proposed combination of materials, parameters, and techniques produces joints with the required properties. Test plates of ZG15Cr1Mo1 (38mm thickness) were prepared with a single-V groove configuration (as shown in the referenced diagram). The welding was performed using the defined FCAW-G process with the parameters meticulously recorded.
| Shielding Gas | Flow (L/min) | Current (A) | Voltage (V) | Travel Speed (mm/min) | Heat Input (kJ/cm) | Preheat & Interpass Temp. (°C) |
|---|---|---|---|---|---|---|
| 80%Ar-20%CO₂ | 15-18 | 250-270 (DCEP) | 27-28 | 300-510 | 10-13 | 250-350 |
The heat input (Q) is a critical derived parameter calculated using the standard formula:
$$ Q = \frac{60 \times Voltage \times Current}{1000 \times Travel Speed} $$
Where travel speed is in mm/min, and Q is in kJ/mm. For example, with a current of 260A, voltage of 27.5V, and speed of 400 mm/min:
$$ Q = \frac{60 \times 27.5 \times 260}{1000 \times 400} = \frac{429,000}{400,000} \approx 1.07 \, \text{kJ/mm} = 10.7 \, \text{kJ/cm} $$
This value falls within the targeted low-to-medium heat input range to control microstructural changes. After welding, the test assembly underwent a two-stage heat treatment: first, a local post-heat at 350°C for 2 hours for immediate hydrogen diffusion (de-hydrogenation), followed by a full furnace stress relief and tempering at 680°C for 8 hours. The resulting welded joints were then subjected to destructive testing. The tensile strength of the weldments exceeded 635 MPa, comfortably above the base metal’s specified minimum of 550 MPa. Furthermore, all side-bend specimens passed without indication of defects, proving the soundness and ductility of the weld metal and fusion line. This successful qualification provided the technical mandate to proceed with production repairs.
The practical application involved the repair of a 350 MW steam turbine IP outer cylinder, a colossal component with a section thickness of 80 mm and a total casting weight exceeding 80 tonnes. The established procedure was translated into a detailed work instruction. The initial and most critical step is the complete removal of the casting defect. This was achieved by machining (e.g., milling, grinding) until the defect was entirely eliminated, verified by liquid penetrant testing (PT). The prepared groove was then preheated to a minimum of 250°C, with the temperature maintained throughout the welding process.
Welding was executed using the qualified parameters, adhering to the principle of low heat input and multi-pass, multi-layer technique. A vital practice employed after depositing each weld bead was immediate peening. This mechanical working of the hot weld metal helps to plastically deform it, thereby mitigating residual tensile stresses that could contribute to cracking—a crucial tactic when dealing with a high carbon equivalent material prone to casting defect repair cracking. Upon completion of welding for a specific defect area, an intermediate local post-weld heat treatment at 350°C for 2 hours was performed to diffuse hydrogen before moving to other areas.
After all identified casting defect on the cylinder were repaired, the entire component underwent a final furnace-based post-weld heat treatment. The cycle followed a controlled ramp-up to 680°C, a soak period of 8 hours to ensure uniform temperature and adequate stress relaxation, followed by a controlled cool-down. This final treatment is essential for achieving a stable microstructure, optimizing mechanical properties, and ensuring the homogeneity of stress distribution across the complex casting geometry. Non-destructive examination (NDE) via ultrasonic testing (UT) of the repaired areas was conducted post-PWHT. The results confirmed that all weld repairs met the stringent acceptance criteria, rendering the cylinder fit for service.
In conclusion, the challenge of casting defect in large, critical components like steam turbine cylinders demands a methodical and science-based approach. It begins with a clear understanding of the base metal’s weldability, quantified by metrics like carbon equivalent. It proceeds through rigorous verification of material properties and systematic selection of a welding process and consumable that enhance both quality and productivity. The cornerstone of the strategy is the execution of a full welding procedure qualification that validates the mechanical and metallurgical soundness of the proposed repair method. Finally, the transfer of this qualified procedure to production must be disciplined, incorporating essential steps like thorough defect removal, controlled preheat/interpass temperatures, in-process stress relief techniques like peening, and a definitive post-weld heat treatment. This holistic framework—from analysis, through qualification, to controlled application—ensures that casting defect are not merely patched but are permanently repaired to restore, and often enhance, the component’s integrity for a long and reliable service life under demanding operational conditions.